1. Field of the Invention
The present invention relates to a radio receiver used in a radio system for non-continuous transmission and a frequency correcting method therefor. The invention particularly relates to a technique used for a modulation scheme such as the frequency shift keying (FSK) modulation scheme or the phase shift keying (PSK) modulation scheme using delay detection in which transmission and reception frequency deviation appears as DC offset in a demodulated (detected) signal, the technique being for removing the DC offset superimposed on the demodulated signal.
2. Description of the Related Art
In radio receivers used in a radio system for non-continuous transmission, when accepting a call, the call acceptance rate may be lowered unless transmission and reception frequency deviation, caused by the reception frequency of the receiving station being shifted from the transmission frequency of a radio transmitter, is promptly detected and corrected.
In particular, in a communication system that often repeats voice communication using the push-to-talk system in which one station performs transmission while another station is performing reception using the same frequency, such as land mobile radio (LMR) devices, unless the voice data is detected and demodulated while accordingly correcting the frequency deviation promptly to extract accurate symbols, a break in the start of speech occurs in which a first portion of the call is lost, and a proper call cannot be made.
For example, in the case of a receiver such as a 400-MHz digital radio using the FSK modulation scheme or the PSK modulation scheme, the demodulation (detection) is performed using differential detection through frequency-voltage conversion. Thus, the demodulated signal that is output is a voltage at a multi-valued (e.g., two-valued or four-valued) level corresponding to a frequency, and the frequency deviation appears as DC offset superimposed on the demodulated signal.
Conventionally, in order to remove the frequency deviation, two techniques, namely high-speed pull-in processing and low-speed pull-in processing have been mainly used.
The high-speed pull-in processing is a technique that subtracts, from a received and demodulated signal, the DC offset obtained from a difference between a DC value of a predetermined number of symbol patterns of the demodulated signal and a known DC value of symbol patterns of a synchronization signal. FIGS. 17A to 17C schematically show the processing.
FIG. 17A shows an original demodulated signal, wherein one call is configured by a plurality of frames each having a predetermined length, and each frame is schematically configured including the synchronization signal including a synchronization word and a preamble, and voice or non-voice data.
In the high-speed pull-in processing, a DC value (DC offset) shown in FIG. 17B is obtained corresponding to the frequency deviation Δf of the demodulated signal, from the portion of the synchronization signal, and the DC value is taken as a correction value, which is to be subtracted from the demodulated signal. Thus, as shown in FIG. 17C, symbols are promptly extracted and the data is demodulated from the first data.
However, according to the high-speed pull-in processing, the reliability of the obtained DC value is low in a noise environment, and, if erroneous correction is performed as it is, bit error may be likely to occur when recovering symbol data.
Furthermore, even when there is frequency deviation, only the DC offset is subtracted from the demodulated signal, and the oscillation frequency of the local oscillator is not corrected. Thus, as shown in FIG. 18, the received signal may be partially lost by a band-limiting band-pass filter at the intermediate-frequency stage.
Accordingly, as shown in FIG. 19, the demodulated signal is distorted, and the reliability of the obtained DC value is lowered, and, thus, the level of precision in establishing the frame synchronization (call acceptance rate) is lowered.
FIG. 18 is a graph showing frequency characteristics of a signal in which a high-frequency signal is subjected to frequency conversion, is retrieved by a digital signal processor for demodulation, is subjected further to orthogonal demodulation and low-frequency conversion, and is about to be subjected to demodulation (symbol recovery), and a band-pass filter.
If there is no frequency deviation (Δf=0), the signal is within the pass bandwidth (12 kHz) of the band-pass filter, but, if there is frequency deviation (Δf=500 Hz toward the high-frequency side in FIG. 18), the signal (the high-frequency side) is partially cut by the band-pass filter and lost. Note that the entire receiver and the demodulator circuit will be described in detail in the embodiment below.
Thus, in consideration of the influence of noise, the correction value obtained from the portion of the synchronization signal including the preamble and the synchronization word is used in a divided manner a plurality of times. FIGS. 20A to 20C schematically show the processing.
As in FIGS. 17A to 17C, FIG. 20A shows an original demodulated signal, FIG. 20B shows a correction value, and FIG. 20C shows a corrected demodulated signal. In FIGS. 20A to 20C, the correction is performed on each frame by ¼ the initially obtained correction value. Accordingly, even in a noise environment, excessive correction may be unlikely to be performed, but a plurality of frames are required before the data is accurately demodulated.
On the other hand, the low-speed pull-in processing is a technique that obtains a DC value, that is, the DC offset from a moving average of the demodulated signal, a moving average of a midpoint of a maximum value and a minimum value of the amplitude of the demodulated signal, or the like, and corrects an oscillation frequency of a local oscillator. FIGS. 21A to 21C schematically show the processing.
As in FIGS. 17A to 17C and 20A to 20C, FIG. 21A shows an original demodulated signal, FIG. 21B shows a correction value, and FIG. 21C shows a corrected demodulated signal.
In FIGS. 21A to 21C, an inaccurate correction value obtained from the portion including the previous preamble and the previous synchronization word is set for data of the first frame, but, as reception of data and reception of the following frames are continued, the correction value is gradually corrected to a proper value due to the moving average. In the second frame, part of the data is erroneous, but the data can be demodulated, and, in the third frame, all data can be accurately demodulated.
According to the low-speed pull-in processing, even in a noise environment, proper correction can be performed, and the occurrence of distortion can be reduced. However, a plurality of frames are required before the data is acquired. Thus, the response is delayed, and the break in the start of speech may possibly occur.
Patent Document 1 (Japanese Patent No. 4835172) has proposed a radio communication device that performs high-speed pull-in processing by obtaining DC offset from a synchronization signal and subtracting it from a demodulated signal, and, as described in paragraph [0025] of this document, when synchronization is established, determines a data bit start position and switches the processing to low-speed pull-in processing for changing an oscillation frequency of a local oscillator according to the DC offset, from that start position (from the data bit of the next frame in the example in FIG. 2 of this document). With this device, as described in paragraph [0026] of this document, the switching of processing at the time when an oscillation frequency of a local oscillator should not be changed, like during data demodulation, is prevented, a deterioration in the response properties such as the break in the start of speech is suppressed, and prompt synchronization establishment and data demodulation are realized.
In Patent Document 1, the DC offset obtained only from the first synchronization signal, used in the high-speed pull-in processing, is used as it is also in the low-speed pull-in processing where the oscillation frequency of the local oscillator is corrected. Accordingly, if the DC offset is large, the oscillation frequency is significantly changed, and, thus, the processing is switched to the low-speed pull-in processing at a time that is not during data demodulation, as described above.
Accordingly, until the processing is switched to the low-speed pull-in processing, the oscillation frequency of the local oscillator is left shifted, and, thus, the received signal is partially lost by a filter and the sensitivity deteriorates as shown in FIG. 18.
In particular, in the case of a weak signal, erroneous operation is caused by noise as described in paragraph [0031] of this document. That is to say, in the case of a weak signal, if noise is superimposed on the synchronization signal, the reliability of the obtained DC offset value is lowered. Thus, the oscillation frequency of the local oscillator is sequentially corrected according to a value obtained by averaging the DC offset for several frames (four frames in FIG. 5).
However, in this low-speed pull-in processing, the DC offset obtained only from the first synchronization signal, used in the high-speed pull-in processing, is used as it is, and, thus, if there is an influence of noise, several frames are required before the average value is obtained in order to eliminate that influence. That is to say, this technique is not significantly different from the case of using only the low-speed pull-in processing. The low-speed pull-in processing obtains an average value from all symbols, and, thus, variations occur depending on data at the beginning of the low-speed pull-in processing, and the level of precision is poor.